This is a 12VDC to 9VDC converter circuit. You may use this DC to DC stepdown converter circuit if you have fixed 12V power supply / battery, but you need 9V power supply for your electronic devices.
R1________ 560 ohm, resistor C1________ 1000uF/16V, Electrolytic capacitor C2________ 100uF/16V, Electrolytic capacitor C3________ 330nF, Ceramic capacitor Z1________ 9.1V, 0.4watt zener diode Q1________ ECG184, NTE184 NPN Transistor
To get a more precise output voltage, replace zener diode Z1 with 10V and R1 with a 1 Kilo ohm potentiometer.
A Coolrib / heatsink for Q1 is optional but recommended for better transformer performance. Simple circuit to power your 9 volt cassette recorder and other stuff.